Makrials Vies 器 www.MaterialsViews.com www.afm-joumal.de A New Approach to Tuning Carbon Ultramicropore Size at Sub-Angstrom Level for Maximizing Specific Capacitance FULL and CO2 Uptake PAPER Jin Zhou,Zhaohui Li,Wei Xing,*Honglong Shen,Xu Bi,Tingting Zhu,Zhipeng Qiu, and Shuping Zhuo* was obtained when the pore size of carbon Ultramicroporous carbon materials with uniform pore size accurately materials matched the dimensions of adjusted to the dimension of electrolyte ions or CO2 molecule are highly the electrolyte ion.With regard to CO, applications.it has been widely efficient ways to fine-tuning ultramicropore size at angstrom level are scarce. A completely new approach to precisely tuning carbon ultramicropore size at Cothe ome sub-angstrom level is proposed herein.Due to the varying activating strength 。 and size of the alkali ions,the ultramicropore size can be finely tuned in the range of 0.60-0.76 nm as the activation ion varies from Litto Cs*.The car. bons prepared by direct pyrolysis of alkali salts of carboxylic phenolic resins yield ultrahigh capacitances of up to 223 Fg-(205 Fcm in ionic liquid diame Puher electrolyte,and superior CO2 uptake of 5.20 mmol g-1 at 1.0 bar and 25 C. studies showed that CO2 uptake is lim- ited by ultramicropores smaller than a Such outstanding performance of the finely tuned carbons lies in its adjust certain diameter at different pressures or able pore size perfectly adapted to the electrolyte ions and CO2 molecule.This temperatures.On the other hand,the work paves the way for a new route to finely tuning ultramicropore size at the volumetric performance is of great impor sub-angstrom level in carbon materials. tance for practical application of carbon materials in supercap citors and co,cap ture.In this sense,high packing density of 1.Introduction 、Crbon mera eh机ati ecent s for materials. with capacitance and CO uptak carbon r electrolyte ions or the CO2 molecule may result in materials diameter smaller than 1.0 nm to be responsible for an anom- exhibiting optimal performance.These impressively show the alous increase in specific capacitance in organic electrolyte importance of uniformly adjusting the ultramicropore size at media.It is suggested that desolvated ions rather than larger- sized solvated ionic species are stored in these ultramicropores. cphyim地me me In the case of ionic liquid (IL)media,maximum capacitance and polymer carbonizationl13 allow production of porous carbon materials with developed porosity.These methods,how- ever,typically lead to poor fine tuning of ultramicropore size hou,H.Shen, 马 at sub-angstrom levels.The most successful synthesis strate gies to control micropore size are based on the selective deal. oving of metal carbides by chlorination.although chlorine gas E-mail:zhuosp_academic@yahoo.com from 0.7t01.0n of ust sofu of He 照 energy-cor suming t tures proces ously hinder large-scaleonmufcnword,efficient ways to precise fine-tuning the ultramicropore size of carbons D010.1002/adfm.201601904 are scarce.It is still very challenging to develop facile approach 0L10o0290010 2016 WILEY-VCH Verlag GmbH Co.KGaA.Weinheim wileyonlinelibrary.com 1
full paper © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 1 was obtained when the pore size of carbon materials matched the dimensions of the electrolyte ion.[4] With regard to CO2 capture applications, it has been widely accepted that CO2 uptake at ambient pressure is determined by the volume of ultramicropores rather than the total pore volume.[5,6] Presser et al. reported that most of the CO2 uptake capacity of carbide derived carbons at ambient pressure to be originated by pores with a diameter smaller than 0.8 nm.[7] Further studies showed that CO2 uptake is limited by ultramicropores smaller than a certain diameter at different pressures or temperatures.[7,8] On the other hand, the volumetric performance is of great importance for practical application of carbon materials in supercapacitors and CO2 capture. In this sense, high packing density of the carbon materials is preferred. As large pore size and broad pore size distribution are usually in contradiction with high packing density, uniform ultramicroporosity are highly desirable for maximizing volumetric capacitance and CO2 uptake for carbon materials. As pointed out above, carbons with uniform and fine tailored ultramicropore size matching the dimensions of the electrolyte ions or the CO2 molecule may result in materials exhibiting optimal performance. These impressively show the importance of uniformly adjusting the ultramicropore size at sub-angstrom levels. Well-established methods such as chemical activation,[9,10] physical activation,[11] template methods,[12] and polymer carbonization[13] allow production of porous carbon materials with developed porosity. These methods, however, typically lead to poor fine tuning of ultramicropore size at sub-angstrom levels. The most successful synthesis strategies to control micropore size are based on the selective dealloying of metal carbides by chlorination, although chlorine gas is very toxic.[14] Hou et al. reported a hot-pressing method to adjust the average pore size of zeolite-templated carbons from 0.7 to 1.0 nm at the expense of using ultrahigh pressures of up to 147 MPa.[15] Cyclic oxidation/thermal desorption treatment allows gradual adjustment of average pore size, although the time- and energy-consuming features of this process seriously hinder large-scale carbon manufacturing.[16] In a word, efficient ways to precise fine-tuning the ultramicropore size of carbons are scarce. It is still very challenging to develop facile approach A New Approach to Tuning Carbon Ultramicropore Size at Sub-Angstrom Level for Maximizing Specific Capacitance and CO2 Uptake Jin Zhou, Zhaohui Li, Wei Xing,* Honglong Shen, Xu Bi, Tingting Zhu, Zhipeng Qiu, and Shuping Zhuo* Ultramicroporous carbon materials with uniform pore size accurately adjusted to the dimension of electrolyte ions or CO2 molecule are highly desirable for maximizing specific capacitance and CO2 uptake. However, efficient ways to fine-tuning ultramicropore size at angstrom level are scarce. A completely new approach to precisely tuning carbon ultramicropore size at sub-angstrom level is proposed herein. Due to the varying activating strength and size of the alkali ions, the ultramicropore size can be finely tuned in the range of 0.60–0.76 nm as the activation ion varies from Li+ to Cs+. The carbons prepared by direct pyrolysis of alkali salts of carboxylic phenolic resins yield ultrahigh capacitances of up to 223 F g-1 (205 F cm-3 ) in ionic liquid electrolyte, and superior CO2 uptake of 5.20 mmol g-1 at 1.0 bar and 25 °C. Such outstanding performance of the finely tuned carbons lies in its adjustable pore size perfectly adapted to the electrolyte ions and CO2 molecule. This work paves the way for a new route to finely tuning ultramicropore size at the sub-angstrom level in carbon materials. DOI: 10.1002/adfm.201601904 Dr. J. Zhou, Z. Li, H. Shen, X. Bi, T. Zhu, Z. Qiu, Prof. S. Zhuo School of Chemical Engineering Shandong University of Technology Zibo 255049, P. R. China E-mail: zhuosp_academic@yahoo.com Prof. W. Xing School of Science State Key Laboratory of Heavy Oil Processing China University of Petroleum Qingdao 266580, P. R. China E-mail: xingwei@upc.edu.cn 1. Introduction Carbon materials have received considerable attention in supercapacitors and CO2 capture.[1] Recent studies have shown that ultramicropores (diameter <0.80 nm) play a key role in determining specific capacitance and CO2 uptake of carbon materials.[2–8] Chmiola et al. pointed out that micropores with a diameter smaller than 1.0 nm to be responsible for an anomalous increase in specific capacitance in organic electrolyte media.[3] It is suggested that desolvated ions rather than largersized solvated ionic species are stored in these ultramicropores. In the case of ionic liquid (IL) media, maximum capacitance Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201601904 www.afm-journal.de www.MaterialsViews.com
5器 Makials www.afm-journal.de Vieus www.MaterialsViews.com to tuning carbon ultramicropore size at sub-angstrom level the nucleophilic addition reaction between phenols and for- when keeping the pore uniformity. maldehyde leads to the formation of hydroxymethyl derivatives Herein,we proposed a completely new strategy to finely that subsequently undergo intermolecular condensation (i.e., tune the pore size of ultramicroporous carbons using different dehydration)to yield methylene and methylene ether bridges. The formation of hydroxymethyl derivatives is very fast at the basic conditions used herein (pH =10).this resulting in a pared by direct carbonization of alkali peeDueoegg2ta large number of primary particles and thus small-sized res porting Information) ns,the ultramic of0.600.76 asheCea The high surface tensi g2actnces1 ng in com rolyte,and s up to 223 F e resulting ting of gel cm id e of 5.20 mmo ga10 (Figure S Supporting I by c and 25 C.To the best of knowledge,these value higher than most of the reported carbon materials.Theo tion at min-)for 2 h in argon.The alkali meta ion-activated carbon materials were finally obtained by washing standing performance of the finely tuned carbons was proved with diluted HCl and deionized water to neutrality.For conven- to lie in its pore size perfectly adjusted to dimensions of the ience,the alkali salts of carboxylic phenolic resin and its corre ions composing the ionic liquids and the CO2 molecule. sponding carbon material are denoted as PR-COOM and MAC, where PR and M stand for phenolic resin and alkali metal ion, respectively. 2.Results and Discussion As illustrated by X-ray energy dispersive spectroscopy (EDS) mapping (Figure S3,Supporting Information).the distribu- 2.1.Formation of the Ultramicroporous Carbon Materials tions of alkali ions are homogeneous.These monodispersed ions as a form of-COOM could produce a homogeneous"in Figure 1 illistrates the preparation of the ultramicroporous situ activation"effect.To study the carbonization process and naterials In a yn 2 4.dibud. acid.alkali hydroxide (MOH). dis. TG-MS)a alysis and in situ x o(XRD)were per med 2a-d sho the TG c nd MS solution. of carboxylic (PR-COOK) between strongly carboxyl groups of 2.4-dihydroxybenzoic acid to form carboxylates ( 2boracaP This solution was then hydrothermally treated at 120 24 h to promote polymerization of phenols with formaldehyde at T<200C could be attributed to the evaporation of mois. resulting in alkali salts of carboxylic phenolic resins.Initially, ture and decarboxylation confirmed by the release of H2o Hydrothermal Drying xeroge M+=Lit,Na',K',Rb*,Cs* Carbonization 250 0.8 ±200 150 pore size of carbons 0 100 2 0.2 activation ion size 0.0 Li Na K Rb Cs 08i Li Na K Rb Cs MAC Figure 1.Sche of the nthesis of MAC. 2016 WILEY-VCH Verlag GmbH Co.KGaA.Weinheim Ady Funct.Mater.2016 D0t10.1002/adfm.201601904
full paper 2 wileyonlinelibrary.com © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim to tuning carbon ultramicropore size at sub-angstrom level when keeping the pore uniformity. Herein, we proposed a completely new strategy to finely tune the pore size of ultramicroporous carbons using different alkali metal ions (Li+, Na+, K+, Rb+, Cs+) as activating agents. Ultramicroporous carbons with highly uniform pores and high packing density were prepared by direct carbonization of alkali salts of carboxylic phenolic resins. Due to the varying activating strength and size of alkali ions, the ultramicropore size can be finely tuned in the range of 0.60–0.76 nm as the activation ion varies from Li+ to Cs+. The resulting carbons present ultrahigh capacitances of up to 223 F g−1 or 205 F cm−3 in ionic liquid electrolyte, and superior CO2 uptake of 5.20 mmol g−1 at 1.0 bar and 25 °C. To the best of knowledge, these values are much higher than most of the reported carbon materials. The outstanding performance of the finely tuned carbons was proved to lie in its pore size perfectly adjusted to dimensions of the ions composing the ionic liquids and the CO2 molecule. 2. Results and Discussion 2.1. Formation of the Ultramicroporous Carbon Materials Figure 1 illustrates the preparation of the ultramicroporous carbon materials. In a typical synthesis, 2,4-dihydroxybenzoic acid, alkali hydroxide (MOH), and formaldehyde were dissolved in deionized water to form a pale yellow homogeneous solution. Immediately, a complete neutral reaction will occur between strongly basic MOH and carboxyl groups (COOH) of 2,4-dihydroxybenzoic acid to form carboxylates (COOM). This solution was then hydrothermally treated at 120 °C for 24 h to promote polymerization of phenols with formaldehyde resulting in alkali salts of carboxylic phenolic resins. Initially, the nucleophilic addition reaction between phenols and formaldehyde leads to the formation of hydroxymethyl derivatives that subsequently undergo intermolecular condensation (i.e., dehydration) to yield methylene and methylene ether bridges. The formation of hydroxymethyl derivatives is very fast at the basic conditions used herein (pH ≈10), this resulting in a large number of primary particles and thus small-sized resin gels (about 200 nm, Figure S1 (Supporting Information)).[17] The high surface tension generated during the hydrothermal process causes the packing texture of gel nanoparticles to collapse, thereby resulting in compacting of gel particles.[18] The resulting red hydrogels were dried to form dark red xerogels (Figure S2, Supporting Information), followed by carbonization at 900 °C (3 °C min−1 ) for 2 h in argon. The alkali metal ion-activated carbon materials were finally obtained by washing with diluted HCl and deionized water to neutrality. For convenience, the alkali salts of carboxylic phenolic resin and its corresponding carbon material are denoted as PR-COOM and MAC, where PR and M stand for phenolic resin and alkali metal ion, respectively. As illustrated by X-ray energy dispersive spectroscopy (EDS) mapping (Figure S3, Supporting Information), the distributions of alkali ions are homogeneous. These monodispersed ions as a form of COOM could produce a homogeneous “in situ activation” effect. To study the carbonization process and activation mechanism, thermogravimetry-mass spectrometry (TG-MS) analysis and in situ X-ray diffraction (XRD) were performed. Figure 2a–d shows the TG curves and MS responses of potassium salt of carboxylic phenolic resin (PR-COOK) and phenolic resin carboxylic acid (PR-COOH). Three gases, including H2O (m/z 18), CO (m/z 28), and CO2 (m/z 44), were detected by mass spectrometry. The little weight loss (<10 wt%) at T < 200 °C could be attributed to the evaporation of moisture and decarboxylation confirmed by the release of H2O Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201601904 www.afm-journal.de www.MaterialsViews.com Figure 1. Schematic diagram of the synthesis of MAC
5器 www.MaterialsViews.con www.afm-journal.de (a) 6 (c) 100日 10 -PR-COOH 0 C0, 90 FULL PR-COO 20 -PR-COOH -PR-COOK 15 H 60 二pR8 PAPER 5 402004006008001000 2004006008001000 Temperature (C) Temperature (C) Ter ure(c) (d) (e) ⑧ Possible activation mechanism (1)PR-C00M-M2C03+C02+H20 800 (2)M2C03→M20+C0 orMC03+C-→M,O+2CO 000 (3)M20+C-M+C0 (4) 400 600 1000 and CO2(Figure 2a-c).In the case of PR-COOH,a significant which are attributed to K(100)(PDF card 01-0500)and KC weight1os of 38.3 wt%in 200-800C is due to th 100)(PDF t of Co w aoo of PR on t observed forme se t the porosi尚i黑hC二eR99 and parti is much more complicated at the temperatures above 200 calate between the carbon layers to form graphite intercalation- The weight loss of about 9.6 wt%in 200-400 C may be due like compounds (e.g.,KCs here)and cause swelling and the to further cross-linking and initial thermal degradation of the disruption of the carbon microstructure,which thereby gener phenolic resin with the formation of KCO,and evolution of ates even more ultramicroporosity.Similar results of TG-MS CO2 and H2O at about 350 C.The weight loss in 400-600 C analyses were obtained for the other alkali salts of carboxylic could be partially attributed to the transformation of K2CO phenolic resins (Figure S4.Supporting Information).indi. into K2O.The sharp weight loss (>20 wt%)above 800 C indi- cating the carbonization process is essentially same for all the cates that the carbon framework is severely etched to form the samples. microporosity. Overall.based on the above observations.a hom Figure 2e presents the in situ XRD patterns at 200.400. 600.and 800C during the olysis of PR-COOK.It can be into alkali car nate (M2CO).H2O.and CO,below 400C.Second.alkali (PDF card71-1466 at around200C.At600℃，the diffra oxides (M,O)are ition of of K2CO onger visible M.CO h nal decomc Third that st of K reaction ed b een transfo ormed K20. We igned a simple tion n of M 0m"s2M+ 20+0 experiment to prove the decomposition of K2CO3.As sho ight loss,t the produced metal es int in Figure S5 (Supporting Information).no precipitates the lattices of the carbon matrix,which is responsible for both were observed when the extract of carbonization residual of stabilization and widening of the interlayer spacing.The PR-COOK at 600C was dropped into BaCl2 solution,indi- interlayer spacing will be primarily determined by the size of cating the absence of CO32-in the residual.This further proves metal intercalate,and this spacing (i.e.,pore size)will be sys- that most K2CO,have been transformed into K2O at 600 C. tematically widened with increasing metal ion size from Lit to Considering the evolution of CO2 and CO at about 520C,the Cs+(Figure S6,Supporting Information).R1 After the removal generation of K2O could be explained by the possible reactions of the intercalated metallic alkali and other alkali compounds of K2C03→K0+Co2orK2C03+C→K0+2C0.When the by washing,the expanded carbon lattices cannot return to their carbonization temperature was increased up to 800C.besides revious nonporous structure and thus achieved to finely tun- the peaks of K,O.new peaks at 10.4 and 16.5 appeared, ble ultramicropore size. 2016 WILEY-VCH Verlag GmbH Co.KGaA,Weinheim
full paper © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 3 and CO2 (Figure 2a–c). In the case of PR-COOH, a significant weight loss of 38.3 wt% in 200–800 °C is due to the continual thermal decomposition of PR-COOH, and a negligible weight loss <0.5 wt% at the last step between 800 and 1000 °C were observed, confirming stable carbon structure was formed above 800 °C. By contrast, the pyrolysis process of PR-COOK is much more complicated at the temperatures above 200 °C. The weight loss of about 9.6 wt% in 200–400 °C may be due to further cross-linking and initial thermal degradation of the phenolic resin with the formation of K2CO3 and evolution of CO2 and H2O at about 350 °C. The weight loss in 400–600 °C could be partially attributed to the transformation of K2CO3 into K2O. The sharp weight loss (>20 wt%) above 800 °C indicates that the carbon framework is severely etched to form the microporosity. Figure 2e presents the in situ XRD patterns at 200, 400, 600, and 800 °C during the pyrolysis of PR-COOK. It can be observed that the PR-COOK began to decompose into K2CO3 (PDF card 71-1466) at around 200 °C. At 600 °C, the diffraction peaks of K2CO3 are no longer visible and K2O (PDF card 26-1327) can be detected, indicating that most of K2CO3 has been transformed into K2O. We further designed a simple experiment to prove the decomposition of K2CO3. As shown in Figure S5 (Supporting Information), no precipitates were observed when the extract of carbonization residual of PR-COOK at 600 °C was dropped into BaCl2 solution, indicating the absence of CO3 2− in the residual. This further proves that most K2CO3 have been transformed into K2O at 600 °C. Considering the evolution of CO2 and CO at about 520 °C, the generation of K2O could be explained by the possible reactions of K2CO3 → K2O + CO2 or K2CO3 + C → K2O + 2CO. When the carbonization temperature was increased up to 800 °C, besides the peaks of K2O, new peaks at 10.4° and 16.5° appeared, which are attributed to K (100) (PDF card 01-0500) and KC8 (100) (PDF card 04-0221), and large amount of CO were detected (Figure 2d). These facts indicate that K2O is reduced by carbon to metallic potassium via the reaction of K2O + C → 2K + CO, and partial carbon atoms were etched into CO to give rise to the porosity.[19] Meanwhile, potassium vapors may intercalate between the carbon layers to form graphite intercalationlike compounds (e.g., KC8 here) and cause swelling and the disruption of the carbon microstructure, which thereby generates even more ultramicroporosity. Similar results of TG-MS analyses were obtained for the other alkali salts of carboxylic phenolic resins (Figure S4, Supporting Information), indicating the carbonization process is essentially same for all the samples. Overall, based on the above observations, a homogeneous “in situ activation” process was presented in Figure 2f. That is: First, alkali salts of phenolic resins decompose into alkali carbonate (M2CO3), H2O, and CO2 below 400 °C. Second, alkali oxides (M2O) are generated via the thermal decomposition of M2CO3 or the redox reaction between M2CO3 and C. Third, framework carbon atoms are etched by M2O via the vigorous redox reaction of M2O + C → 2M + CO, leading to a significant weight loss, then the produced metallic alkali intercalates into the lattices of the carbon matrix, which is responsible for both stabilization and widening of the interlayer spacing.[20] The interlayer spacing will be primarily determined by the size of metal intercalate, and this spacing (i.e., pore size) will be systematically widened with increasing metal ion size from Li+ to Cs+ (Figure S6, Supporting Information).[21] After the removal of the intercalated metallic alkali and other alkali compounds by washing, the expanded carbon lattices cannot return to their previous nonporous structure and thus achieved to finely tunable ultramicropore size. Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201601904 www.afm-journal.de www.MaterialsViews.com Figure 2. a) TG curves, MS responses of evolved gases in TG-MS analysis b) H2O, c) CO2, and d) CO, e) in situ XRD patterns (Cu Ka) during the carbonization of PR-COOK, and f) possible activation mechanism
器 Maknials www.afm-journal.de ViewS www.MaterialsViews.com 200nm 200nm 5 nm g 5μm 200nm 200nm Figure 3.SEM images of a.b)KAC and e.f)CsAC.TEM images of:c.d)KAC and g.h)CsAC. other alkali metals)may be a reason.At 900C Li become leep of carbon Microscopic morphology and pore texture of MAC materials lattices,thus resulting in much more developed porosity.Even were imaged by scanning electron microscopy (SEM)and though the dosage of activation agents used herein was much transmission electron microscopy (TEM)techniques(Figure 3). lower compared to traditional chemical activation processes As revealed by SEM images,the MAC materials were shown (30 wt%of resins as counted in KOH vs 100-400 wt%).the sur- as compact blocks with a smooth surface and no clear pores. face area of KAC material (897 m2g-)was comparable to that However,when observed by TEM,the as-prepared carbons of KOH-activated carbons o2 were found to possess abundant micropores without apparent Dubinin-Radushkevich (D-R)plots from COz adsorption mesopore or macropore signs.High resolution TEM images data provide instructive information about the size of ultrami- ire S7 (Supporting Information)) cropores (diameter <0.8 nm).The slope from the linear D-R plots can be used to estimate micro ore size and uniformity within the MAC structures.These see the Su orting Information for further details).All the from the ho ation duce MAC rials ot liaC exhibited well-defined linear d-r elat 4c-f).In the MAC 1 near fitti ngs f high D-R plo two well-defi ed linear ard type I N2 adsorption a narrow knee at very low relative pressures(P/Po<0.02)and micropore systems(Figure S8,Supporting Information).Due nearly unchanged adsorption amount at higher relative pres- to the weak basicity of LiOH,the neutral reaction between sures.This shape is characteristic of porous materials showing 2,4-dihydroxybenzoic acid and LiOH is incomplete.The exist- a narrow micropore size distribution and a minimal presence ence of unreacted LiOH leads to an uneven dispersion of Lit of mesopores.As shown in Table 1,the SBer versus Smiero and in the phenolic resins,thus resulting in a wide pore size dis- V versus Vmicre values were very similar in all cases,further tribution of LiAC.Apparently.as the activation ions vary from demonstrating the microporous nature of MAC materials. Lit to Cst a systematic widening of micropores takes place Remarkably,the porosity of LiAC is much undeveloped,and the whose size gradually increases from 0.60 up to 0.76 nm.Thus. N,and CO,adsorption data.along with SEM and TEM obser- o-1 resnec vations revealed a purely uniform ultramicropore texture for MAC samples.Remarkably,the ultramicrop re size can be nd si level by the alkali m etal ior anged in react esins.The pared from ergies change( a 0 me red crystalline structure of MAC samples was at 900C with increasingly larger negative values (i.e.higher by XRD (Figure S9a,Supporting Information).XRD profiles reactivity of Mo and carbon)from Li to Cs.Furthermore,the typical of activated carbon materials were obtained with two difference of boiling points for alkali metals (1347 C for Li, broad bands centered at 23.4 and 43 corresponding to the 4 wileyonlinelibrary.com 2016 WILEY-VCH Verlag GmbH Co.KGaA,Weinheim
full paper 4 wileyonlinelibrary.com © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2.2. Structural Characteristics and Chemical Properties of MAC Materials Microscopic morphology and pore texture of MAC materials were imaged by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques (Figure 3). As revealed by SEM images, the MAC materials were shown as compact blocks with a smooth surface and no clear pores. However, when observed by TEM, the as-prepared carbons were found to possess abundant micropores without apparent mesopore or macropore signs. High resolution TEM images (Figure 3c,d,g,h and Figure S7 (Supporting Information)) clearly evidenced highly connected and worm-like micropores within the MAC structures. These pore morphology resulted from the homogeneous “in situ activation” produced by highly dispersed (at atomic level) alkali ions. The porosity properties of the MAC materials were further analyzed by N2 and CO2 adsorption at −196 and 0 °C, respectively. As shown in Figure 4a, all the MAC samples except RbAC presented a standard type I N2 adsorption isotherm with a narrow knee at very low relative pressures (P/P0 < 0.02) and nearly unchanged adsorption amount at higher relative pressures. This shape is characteristic of porous materials showing a narrow micropore size distribution and a minimal presence of mesopores. As shown in Table 1, the SBET versus Smicro and VT versus Vmicro values were very similar in all cases, further demonstrating the microporous nature of MAC materials. Remarkably, the porosity of LiAC is much undeveloped, and the pore volume and the apparent surface areas sharply increased (from 0.07 to 0.70 cm3 g−1 and from 111 to 1312 m2 g−1 , respectively) (Table 1) while varying the activating ion from Li+ to Cs+, thereby revealing activation strength significantly increasing from Li+ to Cs+. The observed trend in activating strengths of alkali ions can be explained by the reactivity difference of M2O and carbon. Gibbs free energies change (ΔG) calculated by Van’t Hoff equation (Table S1, Supporting Information), revealed the reduction process to be more thermodynamically feasible at 900 °C with increasingly larger negative values (i.e., higher reactivity of M2O and carbon) from Li to Cs. Furthermore, the difference of boiling points for alkali metals (1347 °C for Li, <900 °C for other alkali metals) may be a reason. At 900 °C, the alkali metals except Li become metallic vapor with high diffusivity, and could easily intercalate into the deep of carbon lattices, thus resulting in much more developed porosity. Even though the dosage of activation agents used herein was much lower compared to traditional chemical activation processes (30 wt% of resins as counted in KOH vs 100–400 wt%), the surface area of KAC material (897 m2 g−1 ) was comparable to that of KOH-activated carbons.[10,22] Dubinin−Radushkevich (D–R) plots from CO2 adsorption data provide instructive information about the size of ultramicropores (diameter <0.8 nm). The slope from the linear D–R plots can be used to estimate micropore size and uniformity (see the Supporting Information for further details). All the MAC materials except LiAC exhibited well-defined linear D–R plots (correlation coefficient R2 > 0.995, Figure 4c–f). In the case of excellent linear fittings, Dubinin postulated the existence of highly uniform ultramicropores.[6,23] LiAC showed a D–R plot with two well-defined linear ranges, thereby indicating the narrow microporosity to be composed of two micropore systems (Figure S8, Supporting Information). Due to the weak basicity of LiOH, the neutral reaction between 2,4-dihydroxybenzoic acid and LiOH is incomplete. The existence of unreacted LiOH leads to an uneven dispersion of Li+ in the phenolic resins, thus resulting in a wide pore size distribution of LiAC. Apparently, as the activation ions vary from Li+ to Cs+, a systematic widening of micropores takes place, whose size gradually increases from 0.60 up to 0.76 nm. Thus, N2 and CO2 adsorption data, along with SEM and TEM observations revealed a purely uniform ultramicropore texture for MAC samples. Remarkably, the ultramicropore size can be finely and simply adjusted at sub-angstrom level by varying the alkali metal ion exchanged in the parent carboxylic phenolic resins. The pore size tuning resulted from the varying activating strength and ion sizes of the alkali metals from Li+ to Cs+. The crystalline structure of MAC samples was characterized by XRD (Figure S9a, Supporting Information). XRD profiles typical of activated carbon materials were obtained with two broad bands centered at 23.4° and 43° corresponding to the Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201601904 www.afm-journal.de www.MaterialsViews.com Figure 3. SEM images of: a,b) KAC and e,f) CsAC. TEM images of: c,d) KAC and g,h) CsAC
器 www.MaterialsViews.com www.afm-joumal.de (a) (b) a600 -LIAC FULL 500 NaAC- 128 400 10 KAC 300 000 PAPER 200 100 0 0.0 0.20.40.60.81.0 0.00 0.010.020.03 Relative Pressure(P/P) Relative Pressure(P/P (c) (d) 2.5 NaAC 2.0 KAC R=0.9960 R2=0.9995 2.0 L=0.63nm L=0.66nm 1.5 1.5 10 ■ 1.0 24681012 4681012 log'(P/P) log'(P/P) (e) () 2.5 RbAC ·CsAC 2.0 R2=0.9983 R=0.9984 L=0.75nm 2.0 L=0.76nm 10 12 log'(P/P) log'(P /P) Table 1.Textural properties of MAC samples. reflections of the (002)and (101)graphitic planes,respectively. The intensities of these bands decreased from Lit to Cs',in Na sorption COa sorption agreement with the higher activating strength of Cs+that leads to a carbonaceous structure with more defects Raman spectra Sample Img mg cm'em'g lcm'g Inm) (Figure S9b,Supporting Information)showed well-resolved G 111 0.07 0.05 0.0s 0.60/1.17 and D bands centered at 1589 and 1340 cm-.corres sponding NaAC 668 621 035 032 0.36 0.63 to the reflections of the ideal and the disordered graphitic lat- KAC 897 816 0.49 042 0.66 ice, 8)further aterials.X RbAC 1243 1073 0.70 0.55 0.51 0.75 CsAC 1312 1239 0.67 0.63 0.53 0.76 toeochemiclprop Only C and the (ure S1.Supererv rem species were detected size. and deionized water.The oxygen contents of MAC materials 2016 WILEY-VCH Verlag GmbH Co.KGaA.Weinheim wileyonlinelibrary.com 5
full paper © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 5 reflections of the (002) and (101) graphitic planes, respectively. The intensities of these bands decreased from Li+ to Cs+, in agreement with the higher activating strength of Cs+ that leads to a carbonaceous structure with more defects. Raman spectra (Figure S9b, Supporting Information) showed well-resolved G and D bands centered at 1589 and 1340 cm−1 , corresponding to the reflections of the ideal and the disordered graphitic lattice, respectively. The low value of IG/ID (about 0.8) further revealed the amorphous nature of MAC materials. X-ray photoelectron spectroscopy (XPS) measurements were conducted to determine the surface chemical properties of MAC materials (Figure S10, Supporting Information). Only C and O species were detected, thereby revealing complete removal of surface metal species by thorough washing with diluted HCl and deionized water. The oxygen contents of MAC materials Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201601904 www.afm-journal.de www.MaterialsViews.com Figure 4. a) Nitrogen adsorption isotherms. b) CO2 adsorption isotherms. Analysis of the narrow microporosity by Dubinin−Radushkevich equation of: c) NaAC, d) KAC, e) RbAC, and f) CsAC. Table 1. Textural properties of MAC samples. N2 sorption CO2 sorption Sample SBETa) [m2 g−1 ] Smicrob) [m2 g−1 ] VT c) [cm3 g−1 ] Vmicrod) [cm3 g−1 ] V0 e) [cm3 g−1 ] Lf) [nm] LiAC 111 93 0.07 0.05 0.05 0.60/1.17 NaAC 668 621 0.35 0.32 0.36 0.63 KAC 897 816 0.49 0.42 0.42 0.66 RbAC 1243 1073 0.70 0.55 0.51 0.75 CsAC 1312 1239 0.67 0.63 0.53 0.76 a)Brunauer–Emmett–Teller surface area; b)Micropore surface area calculated by the t-plot method; c)Total pore volume; d)Micropore volume calculated by the t-plot method; e)Micropore volume given by CO2 adsorption; f)Average micropore size